In the development of audio equipment exists an ongoing trend towards smaller devices. This is particularly true in the development of listening devices such as for example hearing aids and headsets or similar devices which a user carries with him and which typically are worn in or at the ear or on the head.
In such listening devices, a signal, typically an analog input signal such as for example an acoustic or audio signal is received by means of a microphone or a signal input interface, processed and outputted as an analog output signal to one or more speakers or to a signal output interface. The processing of the analog input signal includes for example analog to digital (A/D) conversion, filtering, amplification, digital to analog (D/A) conversion and may also include other signal processing steps. For carrying out the digital processing, a listening device usually includes corresponding processing means such as for example a microprocessor or a dedicated digital signal processor (DSP).
In order to carry out the above signal processing, energy, typically in the form of electrical energy, is required. For this purpose, the device includes a power supply such as for example an accumulator or a battery that provides the necessary energy. Generally, it can be said that the more signal processing is carried out, the more power is consumed by the device. However, since power supplies typically add a considerable amount to the overall weight and size of the listening device, a tradeoff between the signal processing (power consumption) and the size and weight requirements has to be found.
In currently available listening devices the signal processing includes a step of conversion of the input signal between the time and the frequency domain. The further processing steps, for example a digital filtering, a signal compression or a signal features extraction, are usually carried out subsequently, in the frequency domain. The conversion and the other signal processing steps are usually carried out in blocks at regular time intervals, where each block processing results in a peak of current consumption. These regular or periodic power consumption peaks may cause a corresponding voltage variation and generate an unwanted noise signal at a particular frequency, namely the frequency of the block processing. This noise signal is called undesired periodic noise. Since the undesired periodic noise usually is not sinusoidal, additional undesired periodic noise at higher frequencies (the higher harmonics) is generated as well.
The undesired periodic noise is coupled into the signal processing path mainly through ground and the power supply that is common for all components and particularly for all signal processing subsystems of the listening device.
In order to eliminate or reduce this undesired periodic noise, additional external filters including passive elements such as capacitors, resistors and/or inductors have been introduced. It has for example been proposed to insert a capacitor between the power supply and ground. Another possibility is to insert a filter at the power input of each subsystem.
In listening devices, where the analog input signal is converted into a digital signal with an analog to digital converter (ADC), a further possibility is to add a second ADC the input of which being connected to ground (that allows to measure the noise) and the output of which being subtracted from the output of the signal ADC.
However, all of these known solutions result in additional components of the listening device. These additional components increase not only the size but also the weight and the price of the listening device and therefore contradict the above-mentioned requirements regarding the size and weight of a listening device.